Nico Hotz

Metal nanoparticle direct inkjet printing for low-temperature 3D micro metal structure fabrication

Inkjet printing of functional materials is a key technology toward ultra-low-cost, large-area electronics. We demonstrate low-temperature 3D micro metal structure fabrication by direct inkjet printing of metal nanoparticles (NPs) as a versatile, direct 3D metal structuring approach representing an alternative to conventional vacuum deposition and photolithographic methods. Metal NP ink was inkjet-printed to exploit the large melting temperature drop of the nanomaterial and the ease of the NP ink formulation. Parametric studies on the basic conditions for stable 3D inkjet printing of NP ink were carried out. Furthermore, diverse 3D metal microstructures, including micro metal pillar arrays, helices, zigzag and micro bridges were demonstrated and electrical characterization was performed. Since the process requires low temperature, it carries substantial potential for fabrication of electronics on a plastic substrate.

Next Generation Non-Vacuum, Maskless, Low Temperature Nanoparticle Ink Laser Digital Direct Metal Patterning for a Large Area Flexible Electronics

Flexible electronics opened a new class of future electronics. The foldable, light and durable nature of flexible electronics allows vast flexibility in applications such as display, energy devices and mobile electronics. Even though conventional electronics fabrication methods are well developed for rigid substrates, direct application or slight modification of conventional processes for flexible electronics fabrication cannot work. The future flexible electronics fabrication requires totally new low-temperature process development optimized for flexible substrate and it should be based on new material too. Here we present a simple approach to developing a flexible electronics fabrication without using conventional vacuum deposition and photolithography. We found that direct metal patterning based on laser-induced local melting of metal nanoparticle ink is a promising low-temperature alternative to vacuum deposition– and photolithography-based conventional metal patterning processes. The “digital” nature of the proposed direct metal patterning process removes the need for expensive photomask and allows easy design modification and short turnaround time. This new process can be extremely useful for current small-volume, large-variety manufacturing paradigms. Besides, simple, scalable, fast and low-temperature processes can lead to cost-effective fabrication methods on a large-area polymer substrate. The developed process was successfully applied to demonstrate high-quality Ag patterning (2.1 µΩ·cm) and high-performance flexible organic field effect transistor arrays.

Digital selective growth of ZnO nanowire arrays from inkjet-printed nanoparticle seeds on a flexible substrate.

In this article, we introduce fully digital selective ZnO nanowire array growth on inkjet-printed seed patterning. Through proper natural convection suppression during hydrothermal growth, successful ZnO nanowire local growth can be achieved. Without any need for photolithographic processing or stamp preparation, the nanowire growth location can be easily modified when the inkjet printing process is integrated with a CAD (computer-aided design) system to allow a high degree of freedom when the design needs to be changed. The current proposed process is very fast, low-cost, environmentally benign, and low-temperature. Therefore, it can be applied to a flexible plastic substrate and scaled up for larger substrates for mass production or roll-to-roll processing.

3D micro-structures by piezoelectric inkjet printing of gold nanofluids

3D solid and pocketed micro-wires and micro-walls are needed for emerging applications that require fine-scale functional structures in three dimensions, including micro-heaters, micro-reactors and solar cells. To fulfill this demand, 3D micro-structures with high aspect ratios (>50:1) are developed on a low-cost basis that is applicable for mass production with high throughput, also enabling the printing of structures that cannot be manufactured by conventional techniques. Additively patterned 3D gold micro-walls and -wires are grown by piezoelectric inkjet printing of nanofluids, selectively combined with in situ simultaneous laser annealing that can be applied to large-scale bulk production. It is demonstrated how the results of 3D printing depend on the piezoelectric voltage pulse, the substrate heating temperature and the structure height, resulting in the identification of thermal regions of optimal printing for best printing results. Furthermore a parametric analysis of the applied substrate temperature during printing leads to proposed temperature ranges for solid and pocketed micro-wire and micro-wall growth for selected frequency and voltages.

Controlled free-form fabrication of nanowires by dielectrophoretic dispension of colloids

The vertical growth of conductive gold nanowires in ambient atmosphere through on-demand dielectrophoretically guided deposition of nanoparticle-laden colloids is demonstrated. Accelerated by dielectrophoresis, rapidly evaporating colloidal droplets leave a capillary nozzle (diameter of 500 nm) and are guided through an inhomogeneous electrostatic field. The nanoparticles in the colloidal droplets combine under the action of van der Waals attraction forces on the substrate to form highly flexible vertical nanowires with easily controllable lengths and diameters, here in the range of 150–800 nm. After bending to bridge an electrode pair and after annealing at 300 °C, the sample gold nanowires demonstrate excellent electrical conductivity considering their size.

Simulation of the Postcombustor for the Treatment of Toxic and Flammable Exhaust Gases of a Micro-Solid Oxide Fuel Cell

This work investigates numerically a catalytic postcombustor for a micro-solid oxide fuel cell (SOFC) system. The postcombustor oxidizes toxic and explosive carbon monoxide (CO) and hydrogen exiting a solid oxide fuel cell to carbon dioxide and water. A single 1 mm diameter monolith reactor channel coated with platinum catalyst is modeled in this work. The inlet stream composition is provided by a semi-analytical 2D model of a detailed SOFC system. The model of the postcombustor includes the 2D axisymmetric Navier-Stokes equations, heat conduction in the channel wall, and a multistep finite-rate mechanism for the surface reactions. It is shown that under the operation conditions considered, the influence of homogeneous (gas phase) reactions can be neglected. The model predicts the expected adiabatic temperatures at the postcombustor outlet correctly and can be used for dimensioning and optimization. Postcombustor performance varies significantly with the choice of the operating parameters of the fuel cell. The most critical molecule at the SOFC outlet is shown to be CO because its depletion is slower than that of H 2 for the entire operating range of the SOFC. It can be shown that the postcombustor is able to reduce the level of CO below the toxicity threshold of 25 ppm. Although higher voltages of the fuel cell lead to faster CO conversion in the postcombustor, they also result in a significant increase in wall temperature of the catalyst device. Furthermore, the percentage of SOFC power output used for pump work is lowest for the voltage where the maximum power is reached. For postcombustion the optimal operation point of the SOFC is at the voltage for maximum power of the SOFC system.

Flame-Made CuO/ZnO/Al2O3 Catalyst for Methanol Steam Reforming

In the present study, a catalyst produced by flame spray pyrolysis (FSP) was evaluated for its ability to produce hydrogen-rich gas mixtures. Catalyst particles fabricated by a novel flame spray pyrolysis method resulting in a highly active catalyst with high surface-to-volume ratio were compared to a commercially produced catalyst (BASF F3-01). Both catalysts consisted of CuO/ZnO/Al2O3 of identical composition (CuO 40wt%, ZnO 40wt%, Al2O3 20wt%). Reaction temperatures between 220 and 295 °C, methanol-water inlet flow rates between 2 and 50 μl/min, and reactor masses between 25 and 100 mg were tested for their effect on methanol conversion and the production of undesired carbon monoxide. 100% methanol conversion can be easily achieved within the operational conditions mentioned for this flame-made catalyst — at reactor temperatures of 255 °C (achievable with non-concentrating solar collectors) more than 80% methanol conversion can be reached for methanol-water inlet flow rates as high as 10 μl/min. The FSP catalyst demonstrates similar catalytic abilities as the BASF, produces a consistent gas composition and produces lower overall CO production. Furthermore, the FSP catalyst demonstrates a better suitability to fuel cell use through its higher resistance to degradation and smaller production of carbon monoxide over long-term use.In the present study, the merits of using flame spray pyrolysis to produce CuO/ZnO/Al2O3 methanol steam reforming catalysts are examined, and directly compared to catalysts that are commercially produced in bulk pellet form, and then ground and sieved. The comparison is performed from several different perspectives: catalytic activity and CO production at various temperatures and fuel inlet flow rates; surface and structure characteristics are determined via scanning electron and transmission electron microscopy; surface area characteristics are determined via BET tests.Copyright

Non-Concentrated Solar Collector for Solarthermal Chemical Reactions

The purpose of this study is the proof that non-concentrating solar-thermal collectors can supply the thermal energy needed to power endothermic chemical reactions such as steam reforming of alcoholic (bio-) fuels. Traditional steam reformers require the combustion of up to 50% of the primary fuel to enable the endothermic reforming reaction. Our goal is to use a selective solar absorber coating on top of a collector-reactor surrounded by vacuum insulation. For methanol reforming, a reaction temperature of 220–250°C is required for effective methanol-to-hydrogen conversion. A multilayer absorber coating (TiNOX) is used, as well as a turbomolecular pump to reach ultra-high. The collector-reactor is made of copper tubes and plates and a Cu/ZnO/Al2O3 catalyst is integrated in a porous ceramic structure towards the end of the reactor tube. The device is tested under 1000 W/m2 solar irradiation (using an ABB class solar simulator, air mass 1.5).Numerical and experimental results show that convective and conductive heat losses are eliminated at vacuum pressures of <10−4 Torr. By reducing radiative losses through chemical polishing of the non-absorbing surfaces, the methanol-water mixture can be effectively heated to 240–250°C and converted to hydrogen-rich gas mixture. For liquid methanol-water inlet flow rates up to 1 ml/min per m2 of solar collector area can be converted to hydrogen with a methanol conversion rate above 90%.This study will present the design and fabrication of the solar collector-reactor, its testing and optimization, and its integration into an entire hydrogen-fed Polymer Electrolyte Membrane fuel cell system.Copyright

Novel Non-Concentrated Solar Collector for Solar-Powered Chemical Reactions

The purpose of this study is the proof that non-concentrating solar-thermal collectors can supply the thermal energy needed to power endothermic chemical reactions such as steam reforming of alcoholic (bio-) fuels. Traditional steam reformers require the combustion of up to 50% of the primary fuel to enable the endothermic reforming reaction. Our goal is to use a selective solar absorber coating on top of a collector-reactor surrounded by vacuum insulation. For methanol reforming, a reaction temperature of 220–250°C is required for effective methanol-to-hydrogen conversion. A multilayer absorber coating (TiNOX) is used, as well as a turbomolecular pump to reach ultra-high. The collector-reactor is made of copper tubes and plates and a Cu/ZnO/Al2O3 catalyst is integrated in a porous ceramic structure towards the end of the reactor tube. The device is tested under 1000 W/m2 solar irradiation (using an ABB class solar simulator, air mass 1.5).Numerical and experimental results show that convective and conductive heat losses are eliminated at vacuum pressures of <10−4 Torr. By reducing radiative losses through chemical polishing of the non-absorbing surfaces, the methanol-water mixture can be effectively heated to 240–250°C and converted to hydrogen-rich gas mixture. For liquid methanol-water inlet flow rates up to 1 ml/min per m2 of solar collector area can be converted to hydrogen with a methanol conversion rate above 90%.This study will present the design and fabrication of the solar collector-reactor, its testing and optimization, and its integration into an entire hydrogen-fed Polymer Electrolyte Membrane fuel cell system.Copyright

Novel Nano-Scale Au/α-Fe2O3 Catalyst for the Preferential Oxidation of CO in Biofuel Reformate Gas

Au/α-Fe2O3 catalyst was synthesized using a modified co-precipitation method to generate an inverse catalyst model. The effects of introducing CO2 and H2O during preferential oxidation (PROX) of CO were investigated. The goal of this work was ≥99.8% CO conversion at 80°C. There was an increase in the conversion at all temperatures with the introduction of CO2 and 100% of the CO was converted at the target temperature of 80°C for any amount of CO2. Furthermore, there was an increase in conversion to 100% for water fractions ranging from 3% to 10%.Finally, for realistic conditions of (bio-)fuel reforming, 24% CO2 and 10% water, 99.85% conversion was achieved. A long-term test of 200 hours showed no significant deactivation of the catalyst at a temperature of 80°C in presence of 24% CO2 and 3% water. The mechanism for PROX is not known definitively, however, current literature believes the gold particle size is the key. In contrast, we emphasize the tremendous role of the support particle size.© 2013 ASME